Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of an ELD includes such as an inorganic electroluminescent element and an organic electroluminescent element (hereinafter, referred to as an organic EL element).
An inorganic electroluminescent element has been utilized as a flat light source, however, it requires a high voltage of alternating current to operate an emission element. An organic electroluminescent element is an element provided with a constitution comprising a light emitting layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a positive hole being injected into the light emitting layer to be recombined, resulting emission utilizing light release (fluorescence•phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescent element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
However, in an organic electroluminescence in view of the future practical application, desired has been development of an organic EL element which efficiently emits at a high luminance with a low electric consumption.
In Japanese Patent No. 3093796, a slight amount of a fluorescent substance has been doped in a stilbene derivative, distyrylarylene derivative or a tristyrylarylene derivative, to achieve improved emission luminance and a prolonged lifetime of an element. Further, there are known such as an element having an organic light emitting layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with a slight amount of a fluorescent substance (for example, JP-A 63-264692) and an element having an organic light emitting layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with quinacridone type dye (for example, JP-A 3-255190).
In the case of utilizing emission from an excited singlet as described above, since a generation ratio of a singlet exciton to a triplet exciton is 1/3, that is, a generation probability of an emitting exciton species is 25% and a light taking out efficiency is approximately 20%, the limit of an external quantum efficiency (ηext) of taking out light is said to be 5%.
However, since an organic EL element which utilizes phosphorescence from an excited triplet has been reported from Princeton University (M. A. Baldo et al., Nature vol. 395, pp. 151-154 (1998)), researches on materials exhibiting phosphorescence at room temperature have come to be active.
For example, it is also disclosed in A. Baldo et al., Nature, vol. 403, No. 17, pp. 750-753 (2000), and U.S. Pat. No. 6,097,147.
Since the upper limit of internal quantum efficiency becomes 100% by utilization of an excited triplet, which is principally 4 times of the case of an excited singlet, it may be possible to achieve almost the same ability as a cooled cathode ray tube to attract attention also for an illumination application.
For example, in such as S. Lamansky et al., J. Am. Chem. Soc., vol. 123, p. 4304 (2001), many compounds mainly belonging to heavy metal complexes such as iridium complexes have been synthesized and studied.
Further, in the aforesaid, A. Baldo et al., Nature, vol. 403, No. 17, pp. 750-753 (2000), utilization of tris(2-phenylpyridine)iridium as a dopant has been studied.
An orthometalated complex provided with platinum instead of iridium as a center metal is also attracting attention. With respect to these types of complexes, many examples having a characteristic ligand are known.
Carbazole derivatives such as CBP and m-CP are well know as a host compound for these phosphorescence emitting compounds, (for example; refer to WO 03/80760 and WO 04/74399).
Especially, m-CP or its derivatives having a large band gap are known for a host compound used for blue light emission.
There is disclosed a technology which enables to produce a high luminance light by introduction of a positive hole inhibiting layer (an exciton inhibiting layer). An example of which is disclosed by Pioneer Co. Ltd. In this example, a specific aluminium complex or a fluorinated compound is used to produce an emission with high efficiency (for example, refer to Patent Document 1).
Another example was disclosed in which a pyridine derivative or a pyrimidine derivative was used in an electron transport layer (for example, refer to Patent Documents 2 and 3). Further examples which use a pyridine derivative in an electron transport layer are known (for example, refer to Patent Documents 4 and 5).
However, disclosed technologies are still insufficient for realizing a practical organic electroluminescent element, and further improvement is requested.    Patent Document 1: Unexamined Japanese patent application publication (hereafter it is called as JP-A) 2002-8860    Patent Document 2: WO 06/103909    Patent Document 3: JP-A 2003-45662    Patent Document 4: JP-A 04-68076    Patent Document 5: U.S. Pat. No. 5,077,142